1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to a method and apparatus for regulating optical channel signals with specified wavelengths.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. DWDM (Dense Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. DWDM employs multiple wavelengths or channels in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitating many functions in optical networking.
Transmitting several channels in a single optical fiber requires a process of combining several channel signals at different wavelengths to be multiplexed or interleaving a multiplexed into several channel signals. From a terminology's viewpoint, a device that multiplexes different wavelength channels or groups of channels into one fiber is a multiplexer, and a device that divides the multiplexed channels or groups of channels into individual or subgroups of channels is a demultiplexer. Specifically, when a multiplexer combines two streams of spectrum-periodic optical signals into a single, denser spectrum-periodic signal stream, or in reverse, a demultiplexer separates a single spectrum-periodic signal stream into two complementary spectrum-periodic signal streams, such multiplexer/demultiplexer is called an interleaver.
In general, each of the channel signals comes from a different source and may have transmitted over different mediums, resulting in a different power level. Without equalizing the power levels of the channel signals that are to be combined or multiplexed, some channels in a multiplexed signal may be distorted as a result of various stages of processing the multiplexed signal. On the other hand, many optical devices or systems would not function optimally when incoming signals are beyond a predetermined signal level range. In fact, the power of the incoming signals shall not be too low, neither too high. To ensure that all optical devices or systems receive proper levels of optical signals, attenuation devices are frequently used to adjust the optical signals before they reach an optical device.
Many existing optical attenuation devices lack accuracy and have high feedback noise. Some can not adapt to the power changes in the signals being attenuated or the environmental changes. There is, therefore, a need for techniques capable of dynamically attenuating channel signals.
In WDM and dense WDM (DWDM) systems, it is often needed to select a signal with a particular wavelength from a multiplexed signal with a group of wavelengths. This is advantageous in order to drop/add the same or different channel signals at various points within an optical network. Optical add/drop devices are often employed to add/drop one or more of these channel signals. Thus, it is desirable to have an add/drop device capable of attenuating signals added or dropped.